In such a transmission, which is known from British Pat. No. 2 058 251, the driving-away clutch is a fluid coupling. If such a transmission were to be equipped with an engageable and disengageable plate clutch to serve as the driving-away clutch, operation of this clutch would give rise to problems, inasmuch as it is essential for such a plate-type clutch to engage smoothly under all conditions.
It is an object of the present invention to provide a transmission featuring a plate-type clutch as the driving-away clutch which permits effective operation of the clutch
To this end, the plate-type clutch serving as the driving-away clutch is operated by fluid pressure and the input shaft of the transmission is connected to a second annular channel radially closed from the outside. A second stationary pitot tube terminates in the second annular channel for sensing the pressure of the fluid therein in order to produce a second fluid-pressure signal responsive to the speed of rotation of the input shaft. Also provided are a valve which continuously transmits the higher one of the two fluid-pressure signals and control means for regulating the fluid pressure employed for operation of the wet plate clutch in response to the fluid-pressure signal transmitted.
The use of a pitot tube, known as such, makes it possible in a simple fashion to obtain a hydraulic signal which depends on the angular velocity of the primary pulley (the first fluid-pressure signal) and the input shaft (the second fluid-pressure signal). The second fluid-pressure signal is a measure of the speed of the engine connected to the input shaft and it can be used as a control signal in normal driving away from standstill. In driving away, first the engine speed is increased by the depression of the accelerator pedal. The increased engine speed causes an increase of the second fluid-pressure signal and this signal effects the engagement of the driving-away clutch through the control means. As the vehicle, in driving away, starts moving from standstill the primary pulley will initially rotate slowly - more slowly than the input shaft as long as the driving-away clutch is not yet fully engaged. In consequence, the first fluid-pressure signal is low, in any case lower than the second fluid-pressure signal, so that the valve only transmits the second fluid-pressure signal in order to operate the driving-away clutch.
In driving down a slope, problems may arise if the second fluid-pressure signal is constantly used for operation of the driving-away clutch.
When, during driving, the vehicle speed, and hence the engine speed and also by inference the second fluid-pressure signal, drops to such a level that the driving-away clutch becomes disengaged (disconnected), the engine will run at tickover speed as long as the accelerator is not depressed. Now, if the vehicle goes faster down the slope again without the accelerator pedal being operated, the second fluid-pressure signal remains too low to effect engagement of the driving-away clutch, which would be desirable. The engagement of the driving-away clutch, being of critical importance under those conditions from safety considerations, will then be performed by the first fluid-pressure signal which depending on the vehicle speed will accordingly rise.
According to another feature of the invention, means are provided for influencing the control means in response to the position of the vehicle's accelerator pedal, in such a fashion that upon further depression of the accelerator pedal the engagement of the clutch occurs at a higher engine speed. Deeper depression of the accelerator pedal can be taken as a signal reflecting the wish to drive away faster. Engagement of the driving-away clutch at a higher engine speed promotes a faster take-off.